A1 angst explodes in opposition (7)
I believe the answer is:
against
'opposition' is the definition.
(I've seen this before)
'a1 angst explodes in' is the wordplay.
'aa' becomes 'ai' (I am not sure about this - if you are sure you should believe this answer much more).
'explodes' indicates an anagram.
'in' is an insertion indicator.
'angst' with letters rearranged gives 'agnst'.
'ai' placed inside 'agnst' is 'AGAINST'.
Can you help me to learn more?
(Other definitions for against that I've seen before include "In conflict with" , "antagonistic" , "Versus - in contact with" , "In contrast to; touching" , "close by" .)